The present invention relates to a method for forming images using an imaging sheet which carries a layer of microcapsules containing a photosensitive composition and a color former. More particularly, it relates to a method for forming images by image area-wise transfer of a coating containing said microcapsules and a developer-containing resin to the surface of a sheet of plain paper and to a novel imaging sheet useful therein.
U.S. Pat. No. 4,399,209 to The Mead Corporation describes a transfer imaging system wherein an imaging sheet comprising a support and a layer of microcapsules containing a chromogenic material and a photosensitive composition is image-wise exposed to actinic radiation. The exposed sheet is assembled with a developer sheet and the two are passed through a calender nip whereupon the microcapsules rupture and the contents of the microcapsules are image-wise transferred to the developer sheet where they react and form a colored image.
Typically, the photosensitive composition is a photocurable composition containing an ethylenically unsaturated compound and a photoinitiator. The chromogenic material is a substantially colorless electron-donating color former, and the developer is an electron-acceptor such as an aromatic carboxylic acid salt which is capable of reacting with the color former and forming a visible dye image. Upon image-wise exposing the imaging sheet to actinic radiation, the internal phase of the microcapsules is hardened in the exposed areas. The internal phase remains liquid in the unexposed areas and the internal phase may be hardened to an intermediate degree in areas receiving an intermediate level of exposure. Thus, the microcapsules in the unexposed areas are capable of rupturing and releasing the internal phase upon subjecting the exposed imaging sheet to a uniform rupturing force. The microcapsules in the underexposed areas may rupture and release the internal phase to an intermediate degree, however, in the exposed areas, the hardened photosensitive composition prevents the microcapsules from releasing the internal phase. In this manner, the internal phase of the microcapsules is image-wise released and transferred to the developer sheet where the image is formed.
While the transfer imaging system described in U.S. Pat. No. 4,399,209 can be used in numerous formats and applications, the user usually must purchase two coated paper products to reproduce images; namely, the imaging sheet and the developer sheet. This has several disadvantages. If the imaging sheet and the developer sheet are not designed to be used together, image quality may not be up to standard. More importantly, the developer sheet does not have the appearance, tactility, and other physical properties found in office bond paper. The developer material may also yellow with time. Thus, when an image is formed on a uniformly coated developer sheet, the developer in some cases, over time, detracts from the quality of the image. In principle, these problems can be reduced in copying on plain paper, because the developer is not present in the background areas and, hence, there is less tendency for it to yellow. However, several problems have been encountered in attempts to copy on plain paper. Image quality is not as good as it is in copying on a developer sheet. Plain paper tends to be substantially rougher than developer paper and as a result, images often appear grainy.